Lipopolysaccharides (LPS) from gram-negative bacteria are recognized as the initiators of endotoxic shock which may progress to DIC, ARDS, multiorgan failure and death. However, LPS initiates endotoxic shock only upon its recognition by monocytes or macrophages (MO) with activation of the MO to secrete a variety of injurious products. MO have two pathways for LPS recognition. One recognizes LPS directly, the other, discovered in this laboratory, uses CD14 as a receptor to recognize LPS-LPS binding protein (LBP) complexes. No respond to LPS via the LBP/CD14 pathway at LPS concentrations that are 1000 fold less than necessary for responses by the direct pathway. The MO molecules recognizing LPS by the direct pathway are unknown. It is also unknown whether the direct and LBP/CD14 pathways have any common components, either at the plasma membrane or in their signal transduction pathways. Because they do not have an LBP/CD14 dependent pathway, cells of the murine pre-B cell line 7OZ/3 are models for the direct pathway of MO activation. 7OZ/3 cells respond to LPS with the synthesis of surface immunoglobulin (sIg), without any effect of LBP, and an LPS dose dependency similar to that of the direct MO pathway. It is our hypothesis that direct LPS recognition, like LBP/CD14 LPS recognition, involves LPS recognition by specific cell surface LPS receptors. Preliminary data supporting this hypothesis consists of experiments in which 7OZ/3 cells were photoaffinity labelled with a derivative of S. minnesota Re595 LPS, (125)I -ASD-Re595. If the (125) I-ASD-Re595 is bound to protein, upon photolysis the (125)I becomes covalently bound to the protein, permitting detection of the protein in SDS-PAGE. Performing such an experiment demonstrated the incorporation of (125)I into only two of perhaps hundreds of 7OZ/3 cell membrane proteins. These two proteins are candidates for the 7OZ/3 cell LPS receptor. It is also our hypothesis that when these receptors recognize LPS they utilize a second messenger system shared in part by the LBP/CD14 second messenger system; this may include the kappa-B nuclear factor (NFkappa-B). In 7OZ/3 cells activation of surface immunoglobulin expression includes activation of kappa chain mRNA transcription by NFkappa-B; NFkappa-B is also implicated in MO activation by LPS. Investigation of these hypotheses will provide new knowledge about cellular LPS recognition and response mechanisms as well as provide an opportunity for the development of novel therapeutic measures to combat endotoxic shock.